Salish Sea Ecosystem Conference

Coho Salmon Spawner Mortality in Pacific Northwest Urban Watersheds: Lethal Stormwater Impacts are Prevented by Soil Bioinfiltration

2016SSEC

Adult coho salmon (Oncorhynchus kisutch) return each fall to freshwater spawning habitats throughout western North America. This migration coincides with increasing seasonal rainfall, which in turn increases stormwater runoff, particularly in urban and urbanizing watersheds in the Pacific Northwest because of the land cover characteristics of these basins (e.g., increasing impervious surfaces with increased urbanization). For more than a decade, field assessments in urban streams in the greater Seattle area have shown that adult coho are dying prior to spawning, often at rates exceeding 50% of the entire fall run. The syndrome is characterized by a loss of orientation and equilibrium, leading to death on a time scale of a few hours. Such high levels of mortality are a significant concern for the long term conservation and recovery of wild coho, particularly those distinct population segments vulnerable to ongoing and future development pressures in the Pacific Northwest. Stressors related to temperature, oxygen, and pathogens have been ruled out based upon extensive forensic data. Although indirect evidence from forensic investigations and geospatial land use analyses has implicated toxic runoff as causing the mortality syndrome, this had not been directly demonstrated. Thus, we exposed otherwise healthy coho spawners to undiluted stormwater collected from a high traffic urban arterial (i.e., highway runoff) and highway runoff that was first pre-treated via bioinfiltration through experimental soil columns to remove pollutants. Results revealed that untreated highway runoff collected during nine distinct storm events over three seasons was universally lethal to adult coho relative to unexposed controls. The mortality syndrome was prevented when highway runoff was pretreated by soil infiltration. The findings demonstrate that exposure to urban stormwater is sufficient to cause the adult coho mortality syndrome. However, although the causal chemical stressor(s) have not yet been identified, conventional green stormwater infrastructure (GSI or LID technologies) can effectively protect adult spawners from the acutely toxic effects of highway runoff. Finally, integration of these types of infrastructure may protect salmonid habitat in urban watersheds.